The present invention relates to a perpendicular magnetic recording medium usable as, e.g., a magnetic disk and, more particularly, to a perpendicular magnetic recording medium having desirable recording and reproducing characteristics.
In parallel with the advance of personal computers and work stations, hard disk drives are increasing in capacity and decreasing in size and needs magnetic disks capable of recording data in higher planar density. However, high planar recording density is not achievable with the predominant longitudinal recording system without bringing about the thermal jitter of recorded magnetization ascribable to a decrease in the size of bits. In addition, a coercive force almost exceeding the recording ability of a record head is required. Today, a perpendicular magnetic recording system is in study as an implementation for noticeably increasing planar recording density. For the perpendicular magnetic recording system, a double layer, perpendicular magnetic recording medium has been proposed and is made up of a soft magnetic film having high permeability and a perpendicular magnetization film having high perpendicular anisotropy.
Specifically, a conventional perpendicular recording medium has a soft magnetic film or back layer and a perpendicular magnetization film sequentially formed on a substrate in this order, as taught in the Journal of the Institute of Applied Magnetics Engineers of Japan, Vol. 8, NO. 1, 1984, p. 17. The soft magnetic film and magnetization film may be respectively formed of an NiFe alloy and a CoCr alloy.
However, a problem with the above conventional medium is that the soft magnetic layer or back layer has a domain wall (magnetic domain) structure, e.g., either a Bloch domain wall or a Neil domain wall. The domain wall produces spike noise at the time of recording and reproduction of data from the medium. Spike noise degrades the envelope characteristic of the medium and thereby obstructs faithful reproduction. This kind of noise occurs when a head moves above the domain wall of the soft magnetic film.
Another problem is that the recorded magnetization of the medium is unstable due to external stray magnetic fields. Specifically, the domain wall of the soft magnetic film easily moves due to external stray magnetic fields. If the movement of the domain wall occurs in the region of the soft magnetic film corresponding to the main pole of a perpendicular magnetic head, then the magnetization recorded in the perpendicular magnetization film playing the role of a perpendicular recording layer decreases or practically disappears.
The stray magnetic fields are formed by motors located in the vicinity of a magnetic disk disposed in a magnetic disk drive, e.g., a motor for causing the disk to spin and a motor for positioning the head. Although such magnetic fields are extremely weak, they cause the domain wall of the soft magnetic film to move when concentrating on the tip of the main pole of the head, reducing or even canceling the recorded magnetization. This is a critical defect when it comes to a data storing apparatus.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 3-224122, 5-250651, 5-266455, 6-103553, and 6-180834.